Cheers Ray, that would at least explain why I wasn't getting a return from my search for 'memento'.
My search was for 'memento' (with an 'e'), the file in the startup folder is curiously called 'momento_Startup.htm' (with an 'o'). Weird. (memento_momento.jpg attached). The me-momento error images also attached for your information.
This is in CS3 - which I un/reinstalled - and which now doesn't have a problem (except that Security Assist will still not install correctly - but I have never been able to get SecurityAssist to work on CS3 (sa_error_CS3.jpg) or CS4 (sa_error_CS4.jpg). All steps were taken (winfilecache thru to clean install of DW). I have sort of given up on SA for the time being - but am OK with that as long as I get my other extensions installed OK.
In CS4 - there is no memento/momento file in the config startup folder - but the memento issue persists when there are no extensions installed, and winfilecache has been deleted.
I now understand that that won't be a problem once I get my other WebAssist extensions installed successfully (except for Security Assist!) as they will re-introduce memento/momento and place 'WebAssist Preferences' back into the edit menu.
I will install all but SA over the next day or so and post back if I have further problems - probably on this thread so that you can remember what the issue is - and will also post back on a new thread when I decide that I want Security Assist installed.
Edit PS: The good news is that reinstalling Dreamweaver CS3 has fixed my outstanding ticket regarding no preview available in CSS Sculptor 3 - I now have preview in all areas. Am off to close that ticket!